1. Field of the Invention
This invention relates to a process for reactivating used silver-based ethylene oxide catalysts by first washing the used catalyst and then depositing thereon alkali metal ions.
2. Description of the Prior Art
It is known that the activity of silver catalysts, and particularly their selectivity with respect to the formation of ethylene oxide often decreases during the use of these catalysts in the manufacture of ethylene oxide.
German patent specification No. 2,519,599 published July 1, 1976, discloses a process for the reactivation of used catalysts by impregnating the catalyst with an impregnating solution consisting of 0.1 to 5%w of water, 0.05 to 0.4% of cesium and/or rubidium nitrate, and an aliphatic alcohol having 1-3 carbon atoms, and subsequently evaporating the alcohol at a temperature of 70.degree. to 120.degree. C., optionally by purging with nitrogen, whereby 1 to 1000 ppm of cesium and/or rubidium are deposited on the catalyst.
U.S. Pat. No. 4,033,903 issued July 5, 1977, taught that the selectivity of used silver catalysts is improved by depositing from 0.00004 to 0.008 gram equivalent weights per kilogram (based on the entire catalyst) of ions of one or more of the alkali metals potassium, rubidium or cesium on the catalyst. This is done by impregnating the catalyst with a solution of one or more compounds, for example salts, or these alkali metals in a suitable solvent such as, for example, methanol, ethanol, isopropanol, acetone, methyl acetate or tetrahydrofuran.
However, the improvement of the selectivity obtained by the means taught by German Pat. No. 2,519,599 and U.S. Pat. No. 4,033,903 is not always as high as one would wish. This is particularly the case when the silver catalyst already contains alkali metals, such as for example the catalysts described in U.S. Pat. No. 3,962,136 issued June 8, 1976 and U.S. Pat. No. 4,010,115 issued Mar. 1, 1977 or when the catalyst is partially poisoned with contaminants, such as for example sulphur compounds.
It has now been found that in reactivating such catalysts superior results over the prior art is obtained when the used silver catalyst is first washed with water or a mixture of water with a water-miscible organic solvent before the alkali metals are deposited thereon. This is contrary to the teaching in German Patent Specification No. 2,519,599 for it is stated there that the presence of more than 10% by weight of water in the impregnating solution damages the catalyst. It also appears from Example 20 of the said German patent specification that a used, but still rather active silver catalyst is almost completely deactivated by treating it with an aqueous solution of cesium nitrate.